linux-kernel - Re: spinlock_irqsave() && flags (Was: pm80xx: Spinlock fix)

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Message-ID: <20131223183341.GA6082@redhat.com>
Date:	Mon, 23 Dec 2013 19:33:41 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Jason Seba <jason.seba42@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tomas Henzl <thenzl@...hat.com>, Jack Wang <xjtuwjp@...il.com>,
	Suresh Thiagarajan <Suresh.Thiagarajan@...s.com>,
	Viswas G <Viswas.G@...s.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"JBottomley@...allels.com" <JBottomley@...allels.com>,
	Vasanthalakshmi Tharmarajan 
	<Vasanthalakshmi.Tharmarajan@...s.com>,
	linux-kernel@...r.kernel.org
Subject: Re: spinlock_irqsave() && flags (Was: pm80xx: Spinlock fix)

On 12/23, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@...hat.com> wrote:
>
> > Initially I thought that this is obviously wrong, irqsave/irqrestore
> > assume that "flags" is owned by the caller, not by the lock. And
> > iirc this was certainly wrong in the past.
> >
> > But when I look at spinlock.c it seems that this code can actually
> > work. _irqsave() writes to FLAGS after it takes the lock, and
> > _irqrestore() has a copy of FLAGS before it drops this lock.
>
> I don't think that's true: if it was then the lock would not be
> irqsave, a hardware-irq could come in after the lock has been taken
> and before flags are saved+disabled.

I do agree that this pattern is not safe, that is why I decided to ask.

But, unless I missed something, with the current implementation
spin_lock_irqsave(lock, global_flags) does:

	unsigned long local_flags;

	local_irq_save(local_flags);
	spin_lock(lock);

	global_flags = local_flags;

so the access to global_flags is actually serialized by lock.

> So AFAICS this is an unsafe pattern, beyond being ugly as hell.

Yes, I think the same.

Oleg.

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